The present invention concerns a method of transmitting in successive time slots within a transmission frame, in particular a Time-Division Multiple Access (TDMA) frame, in a radio communication system. The invention relates in particular to the field of time-division multiple access (as opposed to frequency-division multiple access (FDMA) or code-division multiple access (CDMA)) mobile telephony. Of course, it also concerns hybrid systems involving at least time-division multiple access. It is equally applicable to transmission by base stations and to transmission by mobile telephones. It is aimed principally at the field of high bit rate transmission.
The time-division of different communication channels in successive time slots within a common frame is known in itself in the field of radio communications. A frame, for example a Global System for Mobile communications (GSM) frame, conventionally includes eight time slots (TS0 through TS7). A communication channel between a first mobile telephone and a base station is normally assigned a first time slot in a plurality of successive frames. Each transmission channel is also related to a carrier frequency that can be used during the aforementioned time slot. However, the carrier frequency for the same time slot can change from one frame to the next, and therefore for the same transmission channel (the same call between the mobile telephone and the base station), because of an evolving frequency hopping scheme.
As a result of this approach, a carrier frequency could be used by a channel allocated to a first mobile telephone during one time slot and the same carrier frequency could be used by another channel allocated to a second mobile telephone during the next time slot in the same frame. The other channel could be used for a call between the second mobile telephone and the base station covering the geographical area in which the first mobile telephone is located, for example. The second mobile telephone could nevertheless be communicating with another base station.
To prevent signal tail ends resulting from transmission in a first channel during a preceding time slot producing noise degrading the transmission of a message during the next time slot that happens by chance to use the same carrier frequency, one recommendation requires the power of the signal transmitted during a time slot to fall off very rapidly at the end of the slot.
The noise phenomenon referred to above may not be encountered in practice because the recommendation requires that the signal transmitted during the next time slot should be attenuated to a level approximately 36 dB lower than that of the preceding time slot and that the immediately following time slot should not use the same frequency as that used for another call during a preceding time slot.
Nevertheless, the simultaneous activity of more than one operator in the same geographical area, or their freedom of choice in terms of planning of frequency hopping in accordance with the recommendations, can lead to situations in which the same bands are occupied for immediately successive time slots. This leads to tightening of the recommendations to prevent messages transmitted during one time slot generating noise in subsequent time slots.
The attenuation measurement principle to which time-division systems must therefore adhere consists in measuring, during the next time slot, a spectral component shifted (by 400 kHz in GSM, for example) relative to the carrier frequency used in the preceding time slot. This value is 400 kHz because the channels each occupy 200 kHz and the immediately adjoining band is normally not used. Of course, noise occurs at other frequencies, but in order to take all noise into account the recommendation allows the measurement to be carried out during the next time slot at a single frequency, shifted 400 kHz, to reveal the extinction of spurious transmissions and conformance of the equipment to the recommendation.
To conform to this requirement, the power transmitted by the mobile telephone or the base station must rise progressively to its nominal value at the start of each time slot. It must fall to zero at the end of the time slot. However, if they are too sudden, these changes in transmitted power themselves generate noise having a high frequency spectrum that is particularly full. Consequently, too sudden a rise or fall causes transmission of unwanted high frequency harmonics. To solve this problem, there is provision for the nominal transmitted power amplitude to rise in accordance with a progressive curve, for example a cosine curve, over a very short time period. Extinction is similar. The transmission of payload bits is neutralized when the power is rising or falling.
To conform to the recommendation if it is necessary to transmit from the same mobile telephone or the same base station during two successive time slots, in particular if it is necessary to retain transmission at a high bit rate, the nominal transmission level must return to zero in the times between successive time slots. This reduces the usable bit rate because of the neutralization it requires.
The aim of the invention is to minimize interference and therefore to conform to the recommendation, in particular when using two or even three or more successive time slots to transmit a stream of data from the same base station or the same mobile telephone, in particular in a high bit rate application. The frequency can change in successive time slots because of frequency hopping or it can remain the same if frequency planning in the area allows this.
In accordance with the invention, this problem is solved by bringing about, at the time of changing from one time slot to the next, a monotonic progressive variation in the power transmitted by the transmitter, this monotonic variation consequently being different from a fall in power at the end of a time slot followed by a rise in power at the start of the next time slot. The effect of this variation is to return directly to the nominal transmission level assigned to the next time slot during that slot.
Thus transmission in the next time slot is preferably at a higher power than during one or more preceding time slots. This has two consequences: firstly, because the transmitted power is higher, the signals transmitted during the second time slot have better physical protection against noise. Consequently, the encoding to which they are subjected can have a lower level of redundancy, which leads to the usable bit rate in the second time slot increasing compared with the usable bit rate in the first time slot. The second consequence is that a higher nominal power level can be expected in the second time slot although if it had been necessary to reach this nominal power level from zero power, the rising power slope would have been too steep and would have lead to the transmission of prohibited interference.
It can further be shown that less interference is produced and therefore that the recommendation is better complied with by the above approach, i.e. by not requiring the transmitted power to pass through zero but instead by changing via a monotonic step from the transmitted power of the first time slot to the transmitted power of the second time slot.
Consequently, the invention consists in a method of transmitting data in a radio communication system, in a plurality of successive time slots within a transmission frame, in particular a TDMA frame, wherein the power transmitted by a transmitter is modified monotonically between one time slot and the next time slot in the same frame.